A turbofan engine is characterized by a fan that is mounted on a shaft that is turned by a gas turbine. The fan provides thrust, as does the exhaust of the gas turbine. The gas turbine is positioned downstream of the fan, and may include, in order, a low-pressure compressor, a high-pressure compressor, a combustion chamber, a high-pressure turbine, a low-pressure turbine, and an exhaust or core nozzle terminating in a core exit aperture. These components may be enclosed in an inner housing forming a core of the turbofan engine. The fan may be turned by a low-pressure shaft driven by the low-pressure turbine. The high-pressure turbine may be connected to, and thereby rotate, the high-pressure compressor by a high-pressure shaft. The fan is enclosed in an outer housing forming a fan flow duct for directing the thrust produced by the fan and a faired nacelle for directing the free stream around the whole installation. The fan flow duct terminates in a fan nozzle having a fan exit aperture.
There are at least two types of turbofan engine nacelles. A first type includes a fan nozzle in which the fan exit aperture is upstream of the core exit aperture. In order to reduce the noise produced by a turbofan engine of such design, the fan exit aperture may be fitted with generally triangularly shaped “chevrons” that are shaped to extend from the outer surface of the nacelle into the bypass gas flow from the fan exit aperture to create vorticity and thereby increase the amount of mixing between the high velocity air exiting the fan exit aperture and the surrounding free stream air. Increased noise reduction may be accomplished by varying the configuration of the chevrons azimuthally about the circumference of the fan nozzle.
A second type of turbofan engine includes a long-duct, mixed-flow nozzle. In such an engine, the nacelle is extended so that the fan exit aperture is located downstream of the core exit aperture, and is configured to mix a relatively cool bypass gas flow from the fan with the hot gas flow from the core before the gases are exhausted from the turbofan engine. With such a long-duct, mixed-flow nozzle, the inner housing enclosing the core may include a mixer positioned at the core exit aperture. The mixer may include lobes separated by chutes that cooperate to channel gas flow from the bypass duct, defined by the outer housing, with the relatively hot gases of the core exit nozzle from the gas turbine. With such mixed-flow design, the configuration of the lobes and chutes are typically identical, that is, they are radially symmetrical about the central axis of the turbofan engine. Such long-duct, mixed-flow turbofan engines possess increased fuel economy at the expense of weight as compared to the first type of turbofan engine.
As a result of market demand for increasing range and fuel economy, a turbofan engine with a long-duct, mixed-flow nacelle could be used to improve the fuel economy of some turbofan engines without increasing noise, while maintaining the same bypass ratio.